1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor, and further to a display device including the thin film transistor.
2. Description of the Related Art
In recent years, thin film transistors including a thin semiconductor film (with a thickness of several nanometers to several hundreds of nanometers, approximately) over a substrate having an insulating surface (e.g., a glass substrate) have been attracting attention. Thin film transistors are widely used for ICs (integrated circuits) and electronic devices such as electro-optic devices. In particular, development of thin film transistors that are to be used as switching elements in image display devices typified by liquid crystal display devices, EL display devices, and the like is being pushed. In a liquid crystal display device in which potential of pixel electrodes arranged in matrix is controlled with switching elements to form a display pattern in a screen (active matrix liquid crystal display device), specifically, a voltage is applied between a selected pixel electrode and a counter electrode which faces the selected pixel electrode, whereby an orientation of a liquid crystal layer disposed between the selected pixel electrode and the counter electrode is changed, and an optical modulation occurs, which is perceived by a viewer as a display pattern.
Variety of uses for such active matrix liquid crystal display devices has been increasing, and higher size of a screen, definition, and aperture ratio is being demanded. Further, higher reliability has also been required.
As a switching element in a display device, a thin film transistor in which an amorphous semiconductor film or a polycrystalline semiconductor film is used for a channel formation region is widely used; further, a thin film transistor in which a microcrystalline semiconductor film is used is also used. A microcrystalline semiconductor film has higher carrier mobility than an amorphous semiconductor film, and is excellent in electric characteristics. A microcrystalline semiconductor film can be formed by a simpler process than that by which a polycrystalline semiconductor film whose crystallization is carried out by a thermal crystallization method is formed, a laser crystallization method, or the like, and use of the microcrystalline semiconductor film has an advantage of flexibility of a manufacturing process.
It is known that when a microcrystalline semiconductor film is formed over a substrate or an insulating film by a plasma CVD method or the like, an IL (incubation layer; also referred to as a “transition layer”) is formed in a region of the microcrystalline semiconductor film of several nm to 100 nm approximately from an interface with the substrate or the insulating film. Presence of the transition layer, which has low crystallinity, leads to a decrease in electric characteristics of the microcrystalline semiconductor film. In an inverted-staggered thin film transistor, in particular, current flows in or near the transition layer; therefore, a technique has been required which enables formation of a microcrystalline semiconductor film in which generation of the transition layer is suppressed.
As an example of such a technique as described above for forming a crystalline semiconductor film in which generation of the transition layer is suppressed a technique is disclosed in Reference 1 (Japanese Published Patent Application No. 2002-299235). Reference 1 discloses a method for forming a thin semiconductor film: a layer or a thin film including a semiconductor element as a main component is formed over or near a substrate surface; a crystal nucleus is generated while the layer or the thin film including the semiconductor element as the main component is etched; and the crystal nucleus grows to form a crystalline semiconductor film.